Answer to Question #8344 Submitted to "Ask the Experts"

The following question was answered by an expert in the appropriate field:

Q

What is the effect of kVp on x-ray intensity?

A

X-ray intensity increases dramatically with increasing accelerating potential. Theoretically, for machines operating at less than about 500 kVp, if all energy x rays produced in the tube are considered, the intensity, measured by a quantity such as the x-ray energy fluence rate (e.g., keV per cm2 per second) at a fixed point from the machine target, varies as the square of the voltage. Thus, if the voltage is doubled, the energy fluence rate would increase by a factor of four. In actuality, because the lower-energy x rays are removed by the tube walls and by possible added beam filtration, the x-ray intensity often increases even more with voltage, sometimes approaching a dependence on the cube of the voltage.

If exposure rates or dose rates are measured, a similar strong dependence on voltage will be noted, although it will not be exactly the same as the energy fluence rate dependence. This is because, while the dose rate varies directly with energy fluence rate, another factor, the mass energy absorption coefficient, applies in the conversion from energy fluence to dose or exposure, and this factor decreases noticeably with increasing x-ray energy up to about 100 keV and increases slightly beyond 100 keV up to about 500 keV.

If you are concerned with diagnostic x rays, which are generally produced with voltages less than 150 kVp, the above discussion applies. At high x-ray energies that might be used in therapeutic or industrial applications, where accelerating voltages exceed about 500 kVp, electron relativistic effects apply. Many of these higher-energy machines, especially those used in medical therapy (most often linear accelerators), use means other than transformer-generated high voltage to accelerate electrons into targets, and the same energy dependencies as above do not apply. More complex relationships exist, but they will not be discussed here.

If you want more information about this topic you can check various references that deal with the physics of x-ray production. Classic textbooks such as The Physics of Radiology by Johns and Cunningham, 4th ed., Charles C. Thomas, 1983, and Introduction to Radiological Physics and Radiation Dosimetry by F.H. Attix, Wiley and Sons, 1986, may be useful in this regard.

George Chabot, PhD, CHP

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